Zinc and other metal coatings have been in use for many years as an effective means of controlling the corrosion of steel. Due to the protective effect of galvanic coatings, the use of galvanized steel has increased markedly.
In recent years automobile manufacturers, designing lighter weight, corrosion resistant vehicles, have stimulated the use of galvanized steel. A drive to reduce the weight of vehicles calls for the use of high-strength, low-alloy steels with thinner wall thicknesses. The use of thinner sheet steel requires additional corrosion resistance providable through the use of galvanic coatings. Automobile manufacturers are also interested in the forming of coated sheet steel for auto body panels. Forming coated steel requires a relatively thin galvanic coating . Because of the relatively high galvanic and self corrosion of coatings of zinc per se, thick coatings have been necessary. To improve the formability of such protectively coated steel, the thick coating desirably should be replaced with a thin, more corrosion resistant coating with improved galvanic protective properties.
The use of galvanic coatings is increasing in other areas also, such as reinforcing bars for concrete structures. It has been documented that corrosion of steel rebar due to halide salts within the concrete is the basis for the formation of potholes and cracked concrete. Coating the steel with a high performance galvanic coating is one proposal to alleviate this problem.
It is obvious that the increasing use of galvanized steel puts great emphasis on the performance of galvanic coatings. Although pure zinc coatings adequately protect steel, they have several drawbacks including:
(1) The widely differing electrode potentials between zinc and steel [-1.05 Vsce, (Volts versus a saturated calomel reference electrode), and -0.69 Vsce respectively in aerated salt water solutions] result in excessive galvanic corrosion, where the zinc actually overprotects the steel.
(2) Hydrogen gas evolution may occur due to the large cathodic overvoltage. In some instances this may impair the mechanical properties of steel. An example of this is a steel reinforcement bar in concrete.
(3) Painted galvanized steel experiences rapid paint undercutting and delamination due to the excessively large cathodic overvoltage and corrosion product wedging.
(4) The self corrosion rate of zinc is relatively high in certain environments.
The aforementioned problems with zinc coatings are from a corrosion standpoint. Other concerns with coatings in general are: weldability, spangle (grain size), formability, paintability, and brittle metallic layers.
From the foregoing, it is apparent that opportunity exists for considerable improvement over the use of pure zinc coatings for corrosion protection of steel.
The corrosion of pure aluminum coatings has received study. Salama and Thomason (J. Petroleum Tech., Nov., p. 1929, 1984) have found that flame sprayed aluminum (FSA) coatings satisfactorily protected steel in sea water. P. O. Gartland (Paper 299, NACE Corrosion 86 Conference, Houston, Tex., Mar. 17-21, 1986) compared flame sprayed aluminum, and aluminum-3 weight percent magnesium coatings in sea water. There seemed to be little difference between the performance of these two coatings. They both afforded corrosion resistance 10 to 15 times greater than zinc. Thomason (Materials Performance, p. 20, March, 1985) showed that FSA coatings will protect steel. However, if large areas of the coating are damaged, corrosion of the steel may ensue due to the lack of throwing power of aluminum coatings.
Atmospheric corrosion studies by Townsend and Zoccola (Materials Performance, p. 12, October, 1979) indicate that the passivity of aluminum impairs its ability to protect edges. In marine environments, however, the aluminum reportedly protected the edges of the steel. It is believed that the chloride ions keep the aluminum in an active state.
Aluminum is often used as a sacrificial anode for cathodic protection. Reboul et. al. (Corrosion, Vol. 40, No. 7, p. 366, 1984) studied the effect of zinc, indium, tin and mercury on the activation of aluminum. These elements are added to disrupt the aluminum oxide film, thereby activating the aluminum. They have shown that only elements in solid solution are capable of activating the aluminum. D. S. Keir et al (J. Electrochem. Soc., Vol. 116, No. 3, 1969) investigated the addition of bismuth, zirconium, magnesium, silver, cobalt, nickel, and iron to an aluminum-0.1 weight percent tin alloy. They found that only elements that were soluble in the alloy would effect the galvanic current.
It has also been noted that the paintability of aluminum coatings is superior to any zinc or zinc alloy coating (H. Leidheiser, Jr., Corrosion, Vol., 38, No. 5, p. 189, 1983). The reason for this lies in the fact that aluminum is a poor catalyst for the oxygen reduction reaction. The oxygen reduction reaction plays a vital role in the cathodic delamination of paint films.
The present disclosure teaches a unique electrochemical technique for the development of galvanic coating alloys. Through its use, several new galvanic coating alloys containing aluminum and magnesium with improved corrosion resistance have been provided, as will be apparent from what follows.